The field of this invention relates to aromatic polyacyl compounds of polyphenyl structure suitable for polymers useful for forming shaped objects, such as film, fiber and molded parts. The esters are suitable as plasticizers for polyvinylchloride and other polymers.
As is well known, the mechanical and physical properties of a fiber or film depend on the chemical structure of the polymer from which they are made. For example, the melting point, molding temperature, and glass transition temperature of the polymer composition control many of the physical properties and fabrication of the shaped objects. The melting point determines thermal resistance and heat-setting temperature of fibers. Molding temperature determines fabrication temperature. Glass transition temperature (Tg) determines initial modulus, tensile strain recovery, work of recovery, drape and hand, wash-and-wear characteristics, comfort factors and resilience of fibers. The main molecular factors which influence these properties include chain stiffness, the intermolecular forces, orientation and crystallinity.
Accordingly, there has been considerable interest in developing aromatic symmetrical acids as precursors for thermally stable polymers, such as polyesters or polyamides. It is well known that the introduction of aromatic units in the polymer chain backbone results in high bond energies, a low degree of reactivity, and rigidity of the polymer chain structure. The use of aliphatic units in the polymer chain backbone results in flexibility, lower temperature characteristics and decreased strength as compared with the aromatic types.
Substantially all commercial polyester fibers are based on terephthalic acid. While these fibers have many excellent properties there is a need for polyester fibers having a higher Tg than provided by terephthalic acid polyesters. Recently, 2,6-naphthalene dicarboxylic acid has been proposed as a suitable aromatic acid for producing polyesters suitable for tire cord. This acid provides polyesters having a higher Tg than those based on terephthalic acid. For example, poly(ethylene terephthalate) has a Tg of 74.degree. C. while poly(ethylene 2,6-naphthalate) has a Tg of 115.degree.-125.degree. C. However, the difficulties of manufacturing the precursor, i.e., 2,6-dimethylnaphthalene, have made the production of this acid technically difficult and economically costly. The acid can require a four-step synthesis with attendant loss in yield and consequent high cost.
Various other organic polymers have been suggested for use as high temperature fibers, such as copolyamides (Kevlar), polybenzimidazoles, polyoxadiazoles, polyimides and phenylene ring systems (polyphenylenes). Polyarylates and polycarbonates have been suggested for use as engineering plastics. However, all of these are costly and/or difficult to manufacture. Accordingly, there is a need for new aromatic acids suitable for preparing polymers for many uses.
It is the object of this invention to provide a new group of aromatic polycarboxylic acids of polyphenyl structure which will meet this need. Another object of this invention is to provide a process for making these acids. Another object of this invention is to provide a new polycarboxylic acid, specifically 2,2',6,6'- tetramethylbiphenyl-4,4'-dicarboxylic acid, for use in polymer chains. A further object is to provide novel polymers, both polyamides and polyesters, made from these acids. Other and further objects of this invention will be apparent from the following description.
The field of this invention, accordingly, has three aspects. First it relates to novel compositions of matter which are methyl-substituted polyphenylacyl compounds and to the method of preparing these acyl compounds. Second, it relates to novel polyamides based on methyl-substituted polyphenylacyl compounds. Third, it relates to novel polyesters based on these same novel acyl compounds.
These novel methyl-substituted polyphenylacyl compounds (acids, acyl halides, simple esters, e.g., methyl, etc.) are desirable intermediates for producing condensation polymers, such as polyamides and polyesters suitable for shaped articles such as film, fiber and molded parts. The esters of these acids and monohydric alcohols containing 4 to 24 carbon atoms can be used as plasticizers for polyvinylchloride (PVC).
The abstract of an article by Y. Nomura and Y. Takeuchi (J. Chem. Soc. (B) 1970, 956-960) mentions the structure 2,2',6,6'-tetramethylbiphenyl-4,4'-dicarboxylic acid and its methyl ester but no further references to the compounds or to their properties or preparation are given in the abstract or in the article. There is no indication that these compounds were made nor suggestion how to make these compounds. Low yields of other substituted biphenyls are reported by the authors. For example, 4.8 grams of 4,4'-diamino-2,2',6,6'-tetramethylbiphenyl were prepared in 52% yield from 3,5-dimethylnitrobenzene which in turn yielded only 0.10 grams of 4,4'-dicyano-2,2',6,6'-tetramethylbiphenyl in 2% yield, and an overall yield of only 1% to the dicyano compound.
It has been found, in accordance with this invention, that the para carboxylic acid polyphenyls may be prepared in a very convenient manner by the oxidation of the para methyl groups of the methyl-substituted polyphenyls, e.g., bimesityl, by means of molecular oxygen in the presence of a cobalt compound, as for example; cobaltic acetate, and the process is especially convenient and advantageous if carried out in the presence of acetic acid. Ortho-methyl groups, the 2 and 6 methyl substituents of the polyphenyls, were found to require more severe oxidizing conditions; thus the pentacarboxylic biphenyl was found to require more severe oxidative conditions with the utilization of molecular oxygen in the presence of cobalt and manganese acetate, acetic acid, sodium bromide, and tetrabromomethane.
It has been found also that the novel polyesters prepared from the methyl-substituted diphenylcarboxylic acids and dihydric alcohols, such as those having two to ten carbon atoms unexpectedly possess high second order transition temperatures (Tg), transparency, colorlessness and toughness. These novel polyesters can be made into tough films and fibers. Molding temperature of the ethylene glycol polyester is unexpectedly low, with resultant savings in fabrication heat input. Density of the ethylene glycol polyester is 15 to 20% less than the density of poly(ethylene terephthalate), with resultant weight advantage over PET. The limiting oxygen index (LOI) of the ethylene glycol polyester of this invention is approximately 5% more than that of PET with resultant improved non-burning characteristics.
In recent years a limited amount of work has been done involving p,p'-bibenzoic acid and its esters in connection with homopolyesters. U.S. Pat. No. 3,008,929, for example, indicates that this work was not generally fruitful, for a homopolyester of bibenzoic acid and a glycol (e.g., polyethylenebibenzoate) possesses an extremely high melting point making its use in shaped articles entirely impractical, particularly when attempts are made to use it as a film or fiber-forming material. Also, known polybibenzoates exhibit an extremely high rate of crystallization, making orientation of fibers or films therefrom extremely difficult and costly, if not impossible, from a commercial viewpoint. The novel homopolymer polyesters prepared from the methyl-substituted polyphenylcarboxylic acids of this invention, quite unexpectedly in view of the experiences described above, make tough films and fibers with very desirable properties.